The invention relates to a method for diagnosing an exhaust-gas recirculation system (EGR system) wherein the fuel quantity, which is needed for a desired mixture composition, is first approximately computed in a control unit from, inter alia, the stored data (especially of a throughput characteristic line) of the EGR system and, thereafter, is corrected via a control factor which is dependent upon the mixture composition. The mixture composition can be detected by a sensor.
For some time, the exhaust-gas recirculation (EGR) is a known and often applied means for dropping especially the nitrogen oxide emissions of an internal combustion engine.
For this purpose, a defined component flow is taken from the exhaust gas of the engine and supplied to the fresh mixture.
The basis for this is that the exhaust gas of the engine is an inert gas with respect to its essential constituents, that is, a non-reacting gas. The combustion peak temperature is reduced by the admixture of an inert gas to the air/fuel mixture and in this way, the nitrogen oxide emission is reduced.
Electronically controlled EGR systems have been known for a long time which can adjust the optimal exhaust gas recirculation quantity in each operating point of the engine.
It is problematic with respect to all EGR systems that valves and lines become obstructed in the course of time because of deposits of solid substances from the exhaust gas. For this reason, the returned exhaust gas quantities become less with increasing operating time of the engine.
For this reason, it is necessary to monitor the function of an EGR system of this kind.
Furthermore, and because of regulations in the United States of America, all exhaust-gas relevant aggregates/functions of a vehicle having an internal combustion engine must be monitored by its own on-board diagnostic means.
U.S. Pat. No. 5,150,695 discloses a method for diagnosing an EGR system wherein a fault of the system is determined during overrun operation of the vehicle by a fault detecting device. The fault detecting device is for detecting faults of the control arrangement for the EGR system on the basis of a pressure value in the gas intake line when the control valve is in operation for the exhaust-gas recirculation. The fault detecting device also makes the determination of faults on the basis of a second pressure value in the exhaust-gas line when the control valve of the exhaust-gas recirculation is not in operation.
It is disadvantageous with respect to this method for diagnosing the EGR system that, in addition to a pressure sensor in the gas intake line, a further pressure sensor in the exhaust-gas line is required.
Furthermore, no quantitative statement can be obtained with respect to the magnitude of the fault.
It is an object of the invention to provide a method for diagnosing an exhaust-gas recirculation system of the kind described above wherein precise statements as to the operability of the EGR system can be made with the least possible technical complexity.
The method of the invention is for diagnosing an exhaust-gas recirculation system of an internal combustion engine including a control apparatus for controlling the engine. The method includes the steps of: computing approximately a fuel quantity needed for a desired mixture composition in the control apparatus from stored data of the exhaust-gas recirculation system; computing and storing a first control factor while the recirculation system is switched on; computing and storing a second control factor while the recirculation system is switched off; forming the difference between the first and second control factors; comparing the difference to pregiven values; and, if the difference constitutes a departure from the pregiven values, then outputting a fault signal.
The method of the invention has the significant advantage that no additional technical aggregates (such as pressure sensors or other sensors) are needed in order to diagnose the operability of the EGR system because the control factor for controlling the engine must, anyhow, be continuously determined. This advantage is facilitated by the following factors: the computation and storage of the control factor for a switched-in EGR system as well as for a switched-out EGR system; the formation of the difference of these control factors and the comparison of the difference to a pregiven limit value.
An especially advantageous embodiment of the method provides that the amount of the deviation is determined by the control unit and is used as input signal for the control of the engine. A compensation of the effects of deterioration such as because of the above-described deposits, is possible because of this quantitative evaluation of the deviation of the control factors for a switched-in and switched-out EGR system. In this way, especially an adaptive correction of the main load signal and the ignition of the internal combustion engine in an engine control apparatus is possible.
In this way, especially an extension of the service life of the EGR system is possible. Whereas for known EGR diagnostic methods, a fault is already signalized, a fault can at least be compensated to a certain extent over a certain time frame with the described adaptive correction.
In principle, and with an appropriate measuring accuracy as a precondition, the method can be utilized even for very small EGR rates. Preferably, the method is, however, only then carried out when the EGR rate is at least 5%.
Different embodiments are conceivable with respect to the sensors. However, the sensor is advantageously a lambda probe and the control factor is the lambda control factor which operates on the injection time.
In this way, especially precise quantitative statements as to the control factor are possible when the EGR is switched in and switched out.